Abstract

Improved dry matter (DM) digestibility of herbage can be due to changes in fiber composition of leaves and/or stems, and/or to changes in the proportion of leaves and stems in the herbage. Previously, we reported that divergent selection for the ratio of acid detergent lignin (ADL) to cellulose (ADL/CEL) led to differences in timothy (Phleum pratense L.) DM digestibility. Here, we examined plant morphology (leaf-to-stem ratio, plant height and area, and stem diameter), leaf and stem fiber composition, and leaf and stem digestibility in timothy genotypes selected for high or low values of ADL/CEL and in divergent progeny populations derived from these selections. Parental genotypes and their progeny populations were field grown for 2 yr, and measurements were taken at the first harvest of each year. Selection for low values of ADL/CEL reduced ADL and neutral detergent fiber (NDF) concentrations in stems and increased the in vitro true digestibility (IVTD) of stems but had no consistent direct effects on plant morphology or leaf characteristics. Averaged over 2 yr, stems of the low-ADL/CEL group of genotypes had lower ADL and NDF concentrations (by 3.4 and 12 g kg−1 DM, respectively) and a higher IVTD value (by 22 g kg−1 DM) than those of the high ADL/CEL group of genotypes. In addition, stems of the low-ADL/CEL progeny population had lower ADL and NDF concentrations (by 4.2 and 26 g kg−1 DM, respectively) and a higher IVTD value (by 22 g kg−1 DM) than those of the high ADL/CEL progeny population. Thus, the observed changes in overall herbage DM digestibility due to selection for this ratio can be attributed mainly to modification of stem digestibility.

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